Helicopter with anti-torque system, related kit and methods

ABSTRACT

A strake may extend along a portion of an approaching side of a tail boom of a helicopter. A number of vortex generators (VGs) may extend along a portion of a retreating side of the tail boom. For tail booms with circular cross sections, the strake and the VGs are positioned between approximately 5 and 15 degrees below a horizontal plane of the tail boom when viewed end on, on respective sides of the tail boom. For tail booms with non-circular cross sections, the strake and the VGs is positioned between approximately 5 and 15 degrees above a location where a change in curvature is greatest (e.g., where flow separation would otherwise occur) on a bottom half of the tail boom when viewed end on, on respective sides of the tail boom. A fairing may be located on the retreating side on the upper half of the tail boom, to create an asymmetric profile.

BACKGROUND Technical Field

The present disclosure generally relates to helicopters, and morespecifically to anti-torque devices that alter the aerodynamics of ahelicopter.

Description of the Related Art

Traditional single rotor helicopters have a main lifting rotor (“mainrotor”) that provides a lifting force and a tail rotor that provides alaterally directed force used to counter reaction torque of the mainrotor and adjust yaw alignment. The main rotor generates downwash thatflows around the tail boom of the helicopter. It is known to includestrakes on the approaching side of the tail boom to alter the flow ofdownwash from the rotating main rotor so as to generate a compensationforce that at least partially counteracts the reaction torque producedby rotation of the main rotor. The “approaching side” of the tail boomis the side of the tail boom the main rotor blade approaches duringrotation.

For example, U.S. Pat. No. 4,708,305 describes a system for controllingmain rotor torque which reduces the power and size requirements ofconventional anti-torque means (such as a tail rotor). Torque counteringforces are generated by disrupting the main rotor downwash flowingaround the fuselage. In particular, the downward flow is separated fromthe fuselage surface by strakes positioned at specified locations on theapproaching side of the tail boom.

U.S. Pat. No. 8,210,468 describes a stabilizer system for a helicopterthat includes strakes installed on the approaching side of the tail boomand a modified vertical stabilizer. The components of the stabilizersystem cooperate to improve handling of the helicopter (e.g., increasedcross wind tolerance), reduce fatigue (e.g., tail boom fatigue, fuselagefatigue, and the like), improve climb performance, improve cruiseperformance, increase control safety margins, combinations thereof, andthe like.

U.S. Pat. No. 8,985,503 teaches locating strakes at two positions on theapproaching side of the tail boom. A lower strake extends generallydownward from a location of flow separation or maximum change in acurvature of an outer surface on a lower half of the tail boom. An upperstrake extends generally upward from a location of maximum change in acurvature of the outer surface on an upper half of the tail boom. U.S.Pat. No. 8,985,503 also teaches inclusion of a fairing on a retreatingside of the tail boom. The retreating side is the side of the tail boomthat the main rotor retreats or moves away from during rotation. Thefairing is located on the upper half of the tail boom. U.S. Pat. No.8,985,503 further teaches locating a number of vortex generators on theretreating side of the tail boom. The vortex generators are positionedon the fairing, thus extending along a portion of the upper half of thetail boom. U.S. Pat. No. 8,991,747 teaches structures that are similarin some respects to those taught in U.S. Pat. No. 8,985,503.

BRIEF SUMMARY

A helicopter has a tail boom that extends rearwardly from a fuselagesection of the helicopter. The tail boom includes an approaching boomside and retreating or exiting boom side that respectively extendvertically on opposite sides of a vertical plane of the tail boom. Thehelicopter includes a main rotor arranged to pass over the approachingboom side of the tail boom before passing over the retreating or exitingboom side of the tail boom in each of a plurality of rotations of themain rotor in a main rotor rotational direction.

Through analysis and/or testing, applicants have realized that thepresence of strakes, as well as position of strakes on an approachingside of a tail boom can have an advantageous effect on performance.Likewise, applicants have realized that the position of vortexgenerators on the retreating side of a tail boom can have anadvantageous effect on performance. Applicants have further realizedthat including a fairing, along with correctly positioned strakes andvortex generators, may further advantageously effect performance.

The preferred configuration of strake(s), vortex generator, and/orfairing is dependent on the shape of the tail boom.

For tail booms that have a circular cross section, a preferredconfiguration includes a strake located on an approaching side of thetail boom, on a lower half of the tail boom, positioned fromapproximately 5 degrees to 15 degrees below a horizontal plane of thetail boom when viewed along a length of the tail boom. For tail boomsthat have a circular cross section, a preferred configuration includesvortex generators located on a retreating side of the tail boom, on alower half of the tail boom, positioned from approximately 5 degrees to15 degrees below a horizontal plane of the tail boom when viewed along alength of the tail boom. An optional fairing may be located on the upperhalf of the retreating side of the tail boom, for example providing asmooth transition between a tail rotor drive shaft cover and theretreating side of the tail boom, or replacing an existing tail rotordrive shaft cover. Optionally, an upper strake may be employed with tailbooms having a circular cross section. The upper strake may be locatedwithin approximately two inches of where a tail rotor drive shaft coverjoins or intersects the tail boom.

For tail booms that have a non-circular (e.g., elliptical or oblong)cross section, a preferred configuration includes a strake located on anapproaching side of the tail boom, on a lower half of the tail boom,positioned from approximately 5 degrees to 15 degrees above a locationwhere a change in radius of curvature is greatest when viewed along alength of the tail boom, or where flow separation would occur in absenceof the strake. Alternatively, for tail booms that have a non-circular(e.g., elliptical or oblong) cross section, the first strake positionmay be one in which an upstanding leg of the first strake is normal tothe boom surface and from approximately 10 degrees to approximately 16degrees, or more preferably approximately 13 degrees to approximately 14degrees, to the horizontal (e.g., horizontal axis or midplane), wherethe upstanding leg is at an approximately right angle to a base of thefirst strake, which attaches the first strake to the tail boom. For tailbooms that have a non-circular cross section (e.g., elliptical oroblong), a preferred configuration includes vortex generators located ona retreating side of the tail boom, on a lower half of the tail boom,positioned from approximately 5 degrees to 15 degrees above a locationwhere a change in radius of curvature is greatest when viewed along alength of the tail boom, or where flow separation would occur in absenceof the vortex generators. An optional fairing may be located on theupper half of the retreating side of the tail boom, for exampleproviding a smooth transition between a tail rotor drive shaft cover andthe retreating side of the tail boom, or replacing an existing tailrotor drive shaft cover. While an upper strake may be employed withnon-circular tail booms that are not overly elongated, such arepreferably omitted from tail booms. having elongated non-circular crosssections.

A strake positioned on a lower half of the tail boom as describedherein, has an affect similar to that of an extended flap on a wing,increasing pressure on the approaching side relative to pressure on theretreating side of the tail boom. Vortex generators positioned on alower half of the tail boom as described herein, promotes flowattachment from main rotor downwash, and thereby decrease pressure onthe retreating side of the tail boom. Employing a fairing on theretreating side of the tail boom to create an asymmetric profile maypromote flow attachment on the retreating side of the tail boom, and mayimprove the performance and/or stability and/or performance of thehelicopter.

The various counter-torque structures and techniques described hereinmay significantly reduce the amount of power required to drive a tailrotor, may allow reduction in a size of a tail rotor or even a verticalstabilizer, may increase load capacity, and/or provide other advantages.

Strake(s), vortex generators and/or fairing may be installed by anoriginal equipment manufacturer (OEM) or vendor during manufacture of ahelicopter, before sale, lease or delivery to a customer. Alternatively,strake(s), vortex generators and/or fairing may be installed aftermanufacture of a helicopter, for instance after sale, lease or deliveryto a customer. The strake(s), vortex generators and/or fairing may besupplied as an aftermarket kit, which includes instructions forplacement and installation on a tail boom. The instructions may bespecific to make and model of helicopter, or may provide positionspecification information based on a cross-sectional profile of the tailboom.

A helicopter may be summarized as including: a fuselage; a main rotorcoupled to the fuselage and that in operation rotates in a firstrotational direction with respect to the fuselage; an engine carried bythe fuselage and drivingly coupled to rotate the main rotor in the firstrotational direction with respect to the fuselage; a tail boom thatextends rearwardly of the fuselage, the tail boom having an approachingside and a retreating side, the approaching side being a side of thetail boom that the main rotor approaches when rotating in the firstrotational direction, the retreating side being a side of the tail boomthat the main rotor retreats from when rotating in the first rotationaldirection, the retreating side opposite the approaching side across awidth of the tail boom, the approaching side of the tail boom having aconstant radius of curvature about a centerline, and the tail boomhaving an upper half, a lower a half and a horizontal plane that extendsbetween the upper half and the lower half of the tail boom; and a firststrake that extends outwardly from the approaching side of the tailboom, the first strake positioned below the horizontal plane of the tailboom at an angle of from approximately 5 degrees to 15 degrees, theangle measured between the horizontal plane, the centerline and an axisthat extends from the centerline to the first strake.

The helicopter may further include: at least one vortex generator thatextends outwardly from the retreating side of the tail boom, the atleast one vortex generator positioned below the horizontal plane of thetail boom at an angle of from approximately 5 degrees to 15 degrees, theangle measured between the horizontal plane, the centerline and an axisthat extends from the centerline to the at least one vortex generator.The tail boom may have a longitudinal axis, the first strake may be aretrofit strake and extend parallel to the longitudinal axis of the tailboom, and the vortex generator may extend nonparallel to thelongitudinal axis of the tail boom. The helicopter may further include:a tail rotor positioned proximate a distal end of the tail boom and thatin operation rotates in a tail rotor rotational direction; a tail rotordrive shaft drivingly coupled to the tail rotor; and a tail rotor driveshaft cover that extends along the tail boom and which removably coversthe tail rotor drive shaft. The helicopter may further include: a secondstrake that extends outwardly from the approaching side of the tailboom, the second strake positioned proximate a location at which thetail rotor drive shaft cover joins the approaching side of the tailboom. The helicopter may further include: a second strake that extendsoutwardly from the approaching side of the tail boom, the second strakepositioned within two inches above or below a location at which the tailrotor drive shaft cover joins the approaching side of the tail boom. Thehelicopter may further include: a fairing coupled on the retreating sideof the tail boom to create an asymmetry between the approaching and theretreating side of the tail boom. The tail rotor drive shaft cover mayhave an apex, and the fairing may extend from the apex of the tail rotordrive shaft cover down to where the horizontal plane intersects theretreating side of the tail boom. The fairing may provide a smoothtransition between the apex of the tail rotor drive shaft cover and theretreating side of the tail boom. The helicopter may further include: afairing coupled on the retreating side of the tail boom to create anasymmetry between the approaching and the retreating side of the tailboom. The fairing may form a tail rotor drive shaft cover that removablycovers the tail rotor drive shaft. The tail boom may taper in thicknessas the tail boom is traversed from the fuselage to a distal end of thetail boom, the tail boom may extend in a vertical plane of the fuselageat an non-right angle from the fuselage, and the horizontal plane of thetail boom may not be horizontal to a surface of a ground or a horizontalplane of the fuselage.

A method of manufacturing a helicopter having a fuselage, a main rotorcoupled to the fuselage and that in operation rotates in a firstrotational direction with respect to the fuselage, an engine carried bythe fuselage and drivingly coupled to rotate the main rotor in the firstrotational direction with respect to the fuselage, and a tail boom thatextends rearwardly of the fuselage, the tail boom having an approachingside and a retreating side, the approaching side being a side of thetail boom that the main rotor approaches when rotating in the firstrotational direction, the retreating side being a side of the tail boomthat the main rotor retreats from when rotating in the first rotationaldirection, the retreating side opposite the approaching side across awidth of the tail boom, the approaching side of the tail boom having aconstant radius of curvature about a centerline, and the tail boomhaving an upper half, a lower a half and a horizontal plane that extendsbetween the upper half and the lower half of the tail boom, the methodmay be summarized as including: positioning a first strake to extendoutwardly from the approaching side of the tail boom at a first strakeposition, the first strake position located below the horizontal planeof the tail boom at an angle of from approximately 5 degrees to 15degrees, the angle measured between the horizontal plane, the centerlineand an axis that extends from the centerline to the first strakeposition; and fixing the first strake to the approaching side of thetail boom at the first strake position.

The method may further include: positioning at least one vortexgenerator to extend outwardly from the retreating side of the tail boomat a vortex generator position, the at least one vortex generatorposition located below the horizontal plane of the tail boom at an angleof from approximately 5 degrees to 15 degrees, the angle measuredbetween the horizontal plane, the centerline and an axis that extendsfrom the centerline to the vortex generator position; and fixing the atleast one vortex generator to the retreating side of the tail boom atthe vortex generator position. The helicopter may further have a tailrotor positioned proximate a distal end of the tail boom and that inoperation rotates in a tail rotor rotational direction, a tail rotordrive shaft drivingly coupled to the tail rotor; and a tail rotor driveshaft cover that extends along the tail boom and which removably coversthe tail rotor drive shaft, and may further include: positioning asecond strake to extend outwardly from the approaching side of the tailboom at a second strake position, the second strake position locatedwithin two inches above or below a location at which the tail rotordrive shaft cover joins the approaching side of the tail boom; andfixing the second strake to the approaching side of the tail boom at thesecond strake position. The method may further include: positioning afairing on the retreating side of the tail boom at a fairing position tocreate an asymmetry between the approaching and the retreating side ofthe tail boom; and fixing the fairing at the fairing position. Thehelicopter may further have a tail rotor positioned proximate a distalend of the tail boom and that in operation rotates in a tail rotorrotational direction, a tail rotor drive shaft drivingly coupled to thetail rotor; and a tail rotor drive shaft cover that extends along thetail boom and which removably covers the tail rotor drive shaft, thetail rotor drive shaft cover having an apex, and may further include:positioning a fairing on the retreating side of the tail boom at afairing position in which the fairing extends from the apex of the tailrotor drive shaft cover down to where the horizontal plane intersectsthe retreating side of the tail boom. The helicopter may further have atail rotor positioned proximate a distal end of the tail boom and thatin operation rotates in a tail rotor rotational direction, a tail rotordrive shaft drivingly coupled to the tail rotor; and a tail rotor driveshaft cover that extends along the tail boom and which removably coversthe tail rotor drive shaft, the tail rotor drive shaft cover having anapex, and may further include: removing the tail rotor drive shaftcover; and positioning a fairing at a fairing position in which thefairing covers the tail rotor drive shaft and extends down a portion ofthe retreating side of the tail boom.

A retrofit kit for a helicopter may be summarized as including: a firststrake; and instructions that instruct a user to position the firststrake to extend outwardly from an approaching side of a tail boom at afirst strake position, the first strake position located below ahorizontal plane of the tail boom at an angle of from approximately 5degrees to 15 degrees, the angle measured between the horizontal plane,a centerline of the tail boom and an axis that extends from thecenterline to the first strake position.

The retrofit kit may further include: at least one vortex generator,wherein the instructions instruct the user to position the at least onevortex generator to extend outwardly from a retreating side of the tailboom at a vortex generator position, the at least one vortex generatorposition located below the horizontal plane of the tail boom at an angleof from approximately 5 degrees to 15 degrees, the angle measuredbetween the horizontal plane, the centerline and an axis that extendsfrom the centerline to the vortex generator position. The retrofit kitmay further include: a second strake, wherein the instructions instructthe user to position the second strake to extend outwardly from theapproaching side of the tail boom at a second strake position, thesecond strake position located within two inches above or below alocation at which a tail rotor drive shaft cover joins the approachingside of the tail boom. The retrofit kit may further include: a fairing,wherein the instructions instruct the user to position the fairing onthe retreating side of the tail boom at a fairing position to create anasymmetry between the approaching and the retreating side of the tailboom. The instructions may instruct the user to position the fairing toextend from an apex of the tail rotor drive shaft down to where thehorizontal plane of the tail boom intersects the retreating side of thetail boom. The instructions may instruct the user to: remove a tailrotor drive shaft cover; and position the fairing to cover a tail rotordrive shaft and extend part way down the retreating side of the tailboom.

A helicopter may be summarized as including: a fuselage; a main rotorcoupled to the fuselage and that in operation rotates in a firstrotational direction with respect to the fuselage; an engine carried bythe fuselage and drivingly coupled to rotate the main rotor in the firstrotational direction with respect to the fuselage; a tail boom thatextends rearwardly of the fuselage, the tail boom having an approachingside and a retreating side, the approaching side being a side of thetail boom that the main rotor approaches when rotating in the firstrotational direction, the retreating side being a side of the tail boomthat the main rotor retreats from when rotating in the first rotationaldirection, the retreating side opposite the approaching side across awidth of the tail boom, the approaching side of the tail boom having avarying radius of curvature about a centerline, and the tail boom havingan upper half, a lower a half and a horizontal plane that extendsbetween the upper half and the lower half of the tail boom; and a firststrake that extends outwardly from the approaching side of the tailboom, the first strake positioned below the horizontal plane of the tailboom at a position that is from approximately 5 degrees to 15 degreesabove a location at which a change in the radius of curvature of theapproaching side below the horizontal plane is greatest.

The retreating side of the tail boom may have a varying radius ofcurvature about the centerline of the tail boom, and the helicopter mayfurther include: at least one vortex generator that extends outwardlyfrom the retreating side of the tail boom, the at least one vortexgenerator positioned below the horizontal plane of the tail boom at aposition that is from approximately 5 degrees to 15 degrees above alocation at which a change in the radius of curvature of the retreatingside below the horizontal plane is greatest. The tail boom may have alongitudinal axis, the first strake may be a retrofit strake and extendsparallel to the longitudinal axis of the tail boom, and the vortexgenerator may extend nonparallel to the longitudinal axis of the tailboom. The helicopter may further include: a fairing coupled on theretreating side of the tail boom to create an asymmetry between theapproaching and the retreating side of the tail boom. The helicopter mayfurther include: a tail rotor positioned proximate a distal end of thetail boom and that in operation rotates in a tail rotor rotationaldirection; a tail rotor drive shaft drivingly coupled to the tail rotor;and a tail rotor drive shaft cover that extends along the tail boom andwhich removably covers the tail rotor drive shaft. The tail rotor driveshaft cover may have an apex, and the fairing may extend from the apexof the tail rotor drive shaft cover down to where the horizontal planeintersects the retreating side of the tail boom. The fairing may providea smooth transition between the apex of the tail rotor drive shaft andthe retreating side of the tail boom. The helicopter may furtherinclude: a tail rotor positioned proximate a distal end of the tail boomand that in operation rotates in a tail rotor rotational direction; anda tail rotor drive shaft drivingly coupled to the tail rotor, whereinthe fairing forms a tail rotor drive shaft cover that removably coversthe tail rotor drive shaft. The tail boom may taper in thickness as thetail boom is traversed from the fuselage to a distal end of the tailboom, the tail boom may extend in a vertical plane of the fuselage at annon-right angle from the fuselage, and the horizontal plane of the tailboom may not be horizontal to a surface of a ground or a horizontalplane of the fuselage. The radius of curvature of the approaching sidemay be smoothly continuously varying, interrupted only by the firststrake.

A method of manufacturing a helicopter having a fuselage, a main rotorcoupled to the fuselage and that in operation rotates in a firstrotational direction with respect to the fuselage, an engine carried bythe fuselage and drivingly coupled to rotate the main rotor in the firstrotational direction with respect to the fuselage, and a tail boom thatextends rearwardly of the fuselage, the tail boom having an approachingside and a retreating side, the approaching side being a side of thetail boom that the main rotor approaches when rotating in the firstrotational direction, the retreating side being a side of the tail boomthat the main rotor retreats from when rotating in the first rotationaldirection, the retreating side opposite the approaching side across awidth of the tail boom, the approaching side of the tail boom having avarying radius of curvature about a centerline, and the tail boom havingan upper half, a lower a half and a horizontal plane that extendsbetween the upper half and the lower half of the tail boom, the methodmay be summarized as including: positioning a first strake to extendoutwardly from the approaching side of the tail boom at a first strakeposition, the first strake position located below the horizontal planeof the tail boom at a position that is from approximately 5 degrees to15 degrees above a location at which a change in the radius of curvatureof the approaching side below the horizontal plane is greatest; andfixing the first strake to the approaching side of the tail boom at thefirst strake position.

The retreating side of the tail boom may have a varying radius ofcurvature about the centerline of the tail boom, and the method mayfurther include: positioning at least one vortex generator to extendoutwardly from the retreating side of the tail boom at a vortexgenerator position, the at least one vortex generator position locatedbelow the horizontal plane of the tail boom at a position that is fromapproximately 5 degrees to 15 degrees above a location at which a changein the radius of curvature of the retreating side below the horizontalplane is greatest; and fixing the at least one vortex generator to theretreating side of the tail boom at the vortex generator position. Themethod may further include: positioning a fairing on the retreating sideof the tail boom at a fairing position to create an asymmetry betweenthe approaching and the retreating side of the tail boom; and fixing thefairing at the fairing position. The tail rotor drive shaft cover mayhave an apex, and positioning a fairing on the retreating side of thetail boom at a fairing position may include positioning the fairing toextend from the apex of the tail rotor drive shaft cover down to wherethe horizontal plane intersects the retreating side of the tail boom.The method may further include: removing a tail rotor drive shaft cover;and positioning a fairing at a fairing position to cover the tail rotordrive shaft and create an asymmetry between the approaching and theretreating side of the tail boom; and fixing the fairing at the fairingposition.

A retrofit kit for a helicopter may be summarized as including: a firststrake; and instructions that instruct a user to position the firststrake to extend outwardly from the approaching side of the tail boom ata first strake position, the first strake position located below thehorizontal plane of the tail boom at a position that is fromapproximately 5 degrees to 15 degrees above a location at which a changein the radius of curvature of the approaching side below the horizontalplane is greatest.

The retrofit kit may further include: at least one vortex generator,wherein the instructions instruct the user to position the at least onevortex generator to extend outwardly from the retreating side of thetail boom at a vortex generator position, the at least one vortexgenerator position located below the horizontal plane of the tail boomat a position that is from approximately 5 degrees to 15 degrees above alocation at which a change in the radius of curvature of the retreatingside below the horizontal plane is greatest. The retrofit kit mayfurther include: a fairing, wherein the instructions instruct the userto position to create an asymmetry between the approaching and theretreating side of the tail boom. The instructions may instruct the userto position the fairing to extend from an apex of the tail rotor driveshaft cover down to where the horizontal plane of the tail boomintersects the retreating side of the tail boom. The instructions mayinstruct the user to remove a tail rotor drive shaft cover and positionthe fairing to cover the tail rotor drive shaft and extend part way downthe retreating side of the tail boom.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elementsor acts. The sizes and relative positions of elements in the drawingsare not necessarily drawn to scale. For example, the shapes of variouselements and angles are not necessarily drawn to scale, and some ofthese elements are arbitrarily enlarged and positioned to improvedrawing legibility. Further, the particular shapes of the elements asdrawn are not necessarily intended to convey any information regardingthe actual shape of the particular elements, and have been solelyselected for ease of recognition in the drawings.

FIG. 1A is an approaching side elevational view of a helicopter showinga strake, according to at least one illustrated embodiment.

FIG. 1B is a retreating side elevational view of the helicopter of FIG.1A, showing vortex generators.

FIG. 2A is a partial, enlarged, elevation view of a helicopter with thetail boom, showing the approaching side of a tail boom in more detail.

FIG. 2B is a partial, enlarged, elevation view of a helicopter with thetail boom of FIG. 2A, showing the retreating side of a tail boom in moredetail.

FIG. 2C is a cross-sectional view of the tail boom illustrated in FIGS.2A and 2B, taken along section line 2 (FIG. 2A), showing a non-circularprofile of the tail boom and respective positions of the strake andvortex generators on the approaching and retreating sides of the tailboom, respectively.

FIG. 3A is a cross-sectional view of a tail boom similar to thatillustrated in FIGS. 2A-2C, with the additional of a fairing thatreplaces a tail rotor drive shaft cover to create an asymmetric tailboom profile, according to one illustrated embodiment.

FIG. 3B is a cross-sectional view of a tail boom similar to thatillustrated in FIGS. 2A-2C, with the addition of a fairing on theretreating side that extends from a tail rotor drive shaft cover createan asymmetric tail boom profile, according to one illustratedembodiment.

FIG. 4A is a plot of pressure distribution due to downdrafts from a mainrotor passing over a tail boom of a helicopter without strakes or vortexgenerators with a symmetric tail boom profile, the tail boom having anunmodified non-circular or oblong profile.

FIG. 4B is a plot of pressure distribution due to downdrafts from a mainrotor passing over a tail boom of a helicopter which includes a strakeon an approaching side positioned at a first strake position, vortexgenerators on a retreating side of the tail boom positioned at a vortexgenerator position, and a fairing that creates an asymmetric profilesimilar to that of FIG. 3A or 3B, the tail boom having an unmodifiednon-circular or oblong profile.

FIG. 4C is a plot of pressure distribution due to downdrafts from a mainrotor passing over a tail boom of a helicopter without strakes or vortexgenerators with a symmetric tail boom profile, the tail boom having anunmodified circular profile.

FIG. 4D is a plot of pressure distribution due to downdrafts from a mainrotor passing over a tail boom of a helicopter which includes first,second and third strakes on an approaching side, vortex generators on aretreating side of the tail boom positioned at a vortex generatorposition, and a fairing that creates an asymmetric profile similar tothat of FIG. 3A or 3B, the tail boom having an unmodified circularprofile.

FIG. 5 is a cross-sectional view of a tail boom and tail rotor shaftcover of the helicopter, the tail boom having a highly ellipticalcross-sectional profile, which may be modified with a fairing, and whichincludes a strake on an approaching side and vortex generators on aretreating side of the tail boom.

FIG. 6 is a cross-sectional view of a tail boom and tail rotor shaftcover of the helicopter, the tail boom having a highly oblongcross-sectional profile, which may be modified with a fairing, and whichincludes a strake on an approaching side and vortex generators on aretreating side of the tail boom.

FIG. 7 is a cross-sectional view of a tail boom and tail rotor shaftcover of the helicopter, the tail boom having a circular cross-sectionalprofile, which may be modified with a fairing, and which includes firstand second strakes on an approaching side and vortex generators on aretreating side of the tail boom.

FIG. 8 is a flow chart of a method for manufacturing a helicopter ormodifying a tail section of a helicopter, according to one embodiment.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various embodiments of theinvention. However, one skilled in the art will understand that theinvention may be practiced without these details. Tail boom modificationsystems are disclosed in the context of tail sections of helicoptersbecause they have particular utility in this context. However, thesemodifications can be incorporated into other types of aircraft in whichaerodynamics is a significant consideration. Terms, such as “rear,”“front,” “rearward,” “forward,” “counter clockwise,” “clockwise,”“upward,” and “downward,” and variations thereof are used to describethe illustrated embodiments and are used consistently with thedescription of non-limiting exemplary applications. It will beappreciated, however, that the illustrated embodiments can be located ororiented in a variety of desired positions.

As used herein and in the claims, terms such as cross-section,cross-sectional profile, profile and radius of curvature refer to theouter skin of the tail boom, including a fairing if suggested by thecontext, and which does not include any strakes, vortex generators orsimilar structures that extend outward at an abrupt angle from thesurface of the outer skin. The outer skin is typically a closed surface.

Unless the context requires otherwise, throughout the specification andclaims which follow, the word “comprise” and variations thereof, suchas, “comprises” and “comprising” are to be construed in an open,inclusive sense, that is as “including, but not limited to.”

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its broadest sense, that is as meaning “and/or”unless the content clearly dictates otherwise.

The Abstract of the Disclosure provided herein is for convenience onlyand does not interpret the scope or meaning of the embodiments.

FIGS. 1A and 1B show a helicopter 100 including a cabin fuselage section102 and a tail section 104 connected to and extending rearwardly fromthe cabin fuselage section 102. A main rotor 106 is coupled to thefuselage section 102. In operation, the main rotor 106 rotates in afirst rotational direction (indicated by arrow 108) with respect to thefuselage section 102, which provides a lifting force. An engine 110carried by the fuselage section 102 is drivingly coupled to rotate themain rotor 106 in the first rotational direction 108.

The tail section 104 includes a tail boom 112 that extends rearwardly ofthe fuselage section 102. The tail section 104 includes a verticalstabilizer 114 fixedly coupled proximate a distal end 116 of the tailsection 104. A tail rotor 118 is rotatably coupled to the verticalstabilizer 114. In operation, the tail rotor 118 rotates in a tail rotorrotational direction (indicated by arrow 120), that is about an axisthat typically is approximately perpendicular to an axis about which themain rotor 106 rotates. A tail rotor drive shaft cover 124 extends alongat least a portion of a length 126 of the tail boom 112, and whichremovably covers a drive shaft 122 (best illustrated in FIG. 3A-3C). Thedrive shaft 122 is drivingly coupled to the tail rotor 118, for examplefrom the engine 110.

As best illustrated in FIG. 1A, the tail boom 112 has an approachingside 128. The approaching side 128 is the side of the tail boom 112 thatblade of the main rotor 106 approaches when rotating in the firstrotational direction 108. For example, the approaching side 128 may be aleft or port side of the tail boom 112 or fuselage section 102. A firststrake 130 extends along at least a portion the length 126 of the tailboom 112. As described herein, the first strake 130 is positioned ororiented at a first strake position on a lower half of the tail boom112.

As best illustrated in FIG. 1B, the tail boom 112 has a retreating side132. The retreating side 132 is the side of the tail boom 112 that bladeof the main rotor 106 retreats from when rotating in the firstrotational direction 108. For example, the retreating side 132 may be aright or starboard side of the tail boom 150 or fuselage 104. Theretreating side 132 is opposite the approaching side 128 across a width134 of the tail boom 112. A number of vortex generators 136 arepositioned along at least a portion the length 126 of the tail boom 112.As described herein, the vortex generators 136 are positioned ororiented to partially counteract torque produced by the main rotor 106,as at least part of the anti-torque system.

The first strake 130 extends parallel to a longitudinal axis of the tailboom 112. The first strake 130 may take the form of a retrofit strake,or may be installed by the original equipment manufacturer. The vortexgenerators 136 are distributed along the longitudinal axis of the tailboom 112, but are each individually nonparallel to the longitudinal axisof the tail boom 112, for instance at a 45 degree angle, 60 degreeangle, 30 degree angle.

As illustrated in FIGS. 1A and 1B, the tail boom 150 may taper inthickness as the tail boom 150 is traversed from the fuselage to adistal end of the tail boom 150. Typically, the tail boom 150 extends ina vertical plane of the fuselage at an non-right angle from thefuselage, generally angled relatively upwardly from the fuselage towardthe distal end of the tail boom 150. Thus, the horizontal plane of thetail boom 150 is typically not horizontal to a surface of a ground or ahorizontal plane of the fuselage 104.

While not bound by theory, positioning or orienting the first strake 130as described herein, causes the first strake 130 to act in a similarfashion to a flap on a wing, at least partially counteracting torqueproduced by the main rotor 104, as at least part of an anti-torquesystem. The first strake 130 positioned or oriented as described hereinmay advantageously push some of the downwash laterally away from theapproaching side 128, creating a counter force that pushes the tail boom112 toward the retreating side 132 (i.e., in the same direction as tailrotor thrust, into the drawing sheet for FIG. 1A, out of the drawingsheet for FIG. 1B), while also increasing a pressure difference betweenthe approaching side 128 and retreating side 132, increasing a “lift”force that draws the tail boom 112 toward the retreating side 132.

While also not being bound by theory, positioning or orienting thevortex generators 136 as described herein, reenergizes the flow slightlydownstream of a location at which flow would otherwise stagnate inabsence of the vortex generators 136, at least partially counteractingtorque produced by the main rotor 106, as at least part of ananti-torque system. The vortex generators 136 positioned or oriented asdescribed herein may advantageously increase a pressure differencebetween the approaching side 128 and retreating side 132, increasing a“lift” force that draws the tail boom 112 toward the retreating side 132(i.e., in the same direction as tail rotor thrust, into the drawingsheet for FIG. 1A, out of the drawing sheet for FIG. 1B).

FIGS. 2A-2C show a portion of a helicopter 200, according to oneillustrated embodiment.

The helicopter 200 may be similar or even identical to the helicopter100 of FIGS. 1A and 1B. The helicopter 200 includes a tail boom 212 thatextends rearwardly from a cabin fuselage section 202 (FIGS. 2A, 2B). Asbest illustrated in FIG. 2C, the tail boom 212 has a non-circularcross-sectional profile, for instance with shoulders 215 a, 215 b, 215c, 215 d (FIG. 2C). The helicopter 200 includes a tail rotor drive shaft222 that drivingly couples a tail rotor (not illustrated in FIGS. 2A-2C)to an engine (not illustrated in FIGS. 2A-2C) of the helicopter 200. Atail rotor drive shaft cover 224 may be positioned on the tail boom 212to cover the tail rotor drive shaft 222. The tail rotor drive shaftcover may be removably fastened to the tail boom 212, to allowinspection and servicing of the tail rotor drive shaft 222.

A first strake 230 is positioned on an approaching side 228 of the tailboom 212, extending along at least a portion of a length 126 (FIGS. 1A,1B) of the tail boom 212, on a lower half 240 (FIG. 2C) of the tail boom212 at a first strake position 242.

As best illustrated in FIG. 2C, the tail boom 212 may have a verticalaxis or midplane 244 (e.g., plane of symmetry that extends generallyvertically in a frame of reference of the helicopter 200, fuselagesection 202 or ground 246 when on level ground 246). The tail boom 212may have a horizontal axis or midplane 248 (e.g., plane of symmetry thatextends generally horizontally in a frame of reference of the helicopter200, fuselage section 202 or ground 246 when on level ground 246, takinginto account any angle that the tail boom 212 may form with respect tothe fuselage section 202 or any taper in the tail boom 212 as the tailboom 212 is traversed from the fuselage section 202 to a distal end 116(FIGS. 1A, 1B) of the tail boom 212).

For a tail boom 212 with a non-circular cross-sectional profile, thefirst strake position 242 is positioned, oriented and/or located on theapproaching side 228 of the tail boom 212, on a lower half 240 of thetail boom 212, positioned from approximately 5 degrees to 15 degreesabove a location 250 where a change in radius of curvature of thesurface of the approaching side 228 is greatest when viewed along alength of the tail boom 212, or where flow separation would occur inabsence of the first strake 230. An exemplary angle between the location250 of greatest change in radius of curvature on the lower half 240 ofthe approaching side 228, a center location 252 (e.g., where verticaland horizontal midplanes 244, 248 intersect), and the first strakeposition 242 is best illustrated between axes 254, 256 in FIG. 2C. Theray 254 corresponds to a 15 degree angle. The ray 255 corresponds to a 5degree angle.

Alternatively, for a tail boom 212 with a non-circular cross-sectionalprofile, the first strake position 242 is positioned, oriented and/orlocated on the approaching side 228 of the tail boom 212, on a lowerhalf 240 of the tail boom 212, such that an upstanding leg of the firststrake 230 is normal to the boom surface and from approximately 10degrees to approximately 16 degrees, or more preferably approximately 13degrees to approximately 14 degrees to the horizontal (e.g., horizontalaxis or midplane 248), where the upstanding leg is at an approximatelyright angle to a base of the first strake 230, which attaches the firststrake 230 to the tail boom 212. The term approximately when used inconjunction with angles or degrees includes variations of plus or minus3 degrees.

A number of vortex generators 236 are positioned on an retreating side232 of the tail boom 212, extending along at least a portion of a length126 (FIGS. 1A, 1B) of the tail boom 212, on a lower half 240 (FIG. 3C)of the tail boom 212 at a vortex generator position 258.

For a tail boom 212 with a non-circular cross-sectional profile, thevortex generators 236 are positioned, oriented and/or located on theretreating side 232 of the tail boom 212, on the lower half 240 of thetail boom 212, positioned from approximately 5 degrees to 15 degreesabove a location 260 where a change in radius of curvature of thesurface of the retreating side 232 is greatest when viewed along alength of the tail boom 212, or where flow separation would occur inabsence of the vortex generators 236. An exemplary angle between thelocation 260 of greatest change in radius of curvature on the lower half240 of the retreating side 232, a center location 252 (e.g., wherevertical and horizontal midplanes 244, 248 intersect), and the vortexgenerator position 258 is best illustrated between axes 262, 264 in FIG.2C. The ray 262 corresponds to a 15 degree angle. The ray 263corresponds to a 5 degree angle.

FIG. 3A shows a portion of a helicopter 300 a, according to oneillustrated embodiment.

The helicopter 300 a may be similar or even identical to the helicopter100 of FIGS. 1A and 1B. The helicopter 300 a includes many of the sameor similar structures to that illustrated in FIGS. 2A-2C. Identical orsimilar are denominated with the same reference numbers as employed inFIGS. 2A-2C. Some structures are not specifically called out in FIG. 3Ato enhance drawing legibility. Only significant differences with that ofFIGS. 2A-2C are discussed below.

In addition to the first strake 230 and vortex generators 236, thehelicopter 300 a optional includes a fairing 370 a that extends along atleast a portion of the length 126 (FIGS. 1A, 1B) of the tail boom 212.For example, the fairing 370 a may extend along a portion of the tailboom 212 that experiences rotor wash from a main rotor 106 (FIGS. 1A,1B). The combination of the fairing 370 a and the tail boom 212 resultsin the tail section 104 (FIGS. 1A, 1B) having an asymmetricalcross-sectional profile across the vertical axis or midplane 244. Thisasymmetrical cross-sectional profile is shaped to produce lift on theretreating side 232, thus the fairing 370 a at least partiallycounteracts main rotor torque produced by the main rotor 106 (FIGS. 1A,1B), as at least part of the anti-torque system. The fairing 370 a maybe positioned to cover the tail rotor drive shaft 222. For example, thefairing 370 a may be a replacement for a tail rotor drift shaft cover224 (FIG. 3A) that came with the helicopter 300 a. The fairing 370 b canextend at least part way down the retreating side 232 of the tail boom212, providing a smooth transition therewith. The fairing 370 b can besupplied as an aftermarket kit, along with strake(s) 230 and/or vortexgenerators 236, and instructions 371 for installing such at the variouslocations described herein. Alternatively, the fairing 370 a may be partof the helicopter 300 a as manufactured and/or delivered by an OEM.

Optionally, a second strake 372 is positioned on the approaching side228 of the tail boom 212, extending along at least a portion of a length126 (FIGS. 1A, 1B) of the tail boom 212, on an upper half 374 of thetail boom 212 at a second strake position 376. The second strakeposition 376 is preferably within a few inches of where the fairing 370a joins the tail boom 212 on an upper portion 378 of the tail boom 212.The second strake 372 may be employed with tail booms having a circularcross-sectional profile or almost circular cross-sectional profiles(e.g., FIG. 3A), and which are not overly elongated or overly elliptical(e.g., FIG. 6). Applicants have found it advantageous to omit a secondstrake 372, at least for tail booms with overly elongated or overlyelliptical cross-sectional profiles. The second strake 372 adds weight,and tends to create a down force penalty, so any advantage in producingflow separation on the approaching side 228 is at least partially, ifnot wholly, offset.

FIG. 3B shows a portion of a helicopter 300 b, according to oneillustrated embodiment.

The helicopter 300 b may be similar or even identical to the helicopter100 of FIGS. 1A and 1B. The helicopter 300 b includes many of the sameor similar structures to that illustrated in FIGS. 2A-2C. Identical orsimilar are denominated with the same reference numbers as employed inFIGS. 2A-2C, and 3A. Some structures are not specifically called out inFIG. 3B to enhance drawing legibility. Only significant differences withthat of FIGS. 2A-2C and 3A are discussed below.

In addition to the first strake 230 and vortex generators 237, thehelicopter 300 a optional includes a fairing 370 b that extends along atleast a portion of the length 126 (FIGS. 1A, 1B) of the tail boom 212.For example, the fairing 370 b may extend along a portion of the tailboom 212 that experiences main rotor wash from a main rotor 106 (FIGS.1A, 1B). The combination of the fairing 370 b and the tail boom 212results in the tail section 104 (FIGS. 1A, 1B) having an asymmetricalcross-sectional profile across the vertical axis or midplane 244. Thisasymmetrical cross-sectional profile is shaped to produce lift on theretreating side 232, thus the fairing 370 b at least partiallycounteracts main rotor torque produced by the main rotor 106, as atleast part of the anti-torque system.

In contrast to the fairing 370 a (FIG. 3A), the fairing 370 b may bepositioned adjacent the tail rotor drive shaft cover 222. For example,the fairing 370 b may extend from a vertical apex 380 of the tail rotordrive shaft cover 222, down at least a portion of the retreating side232 of the tail boom 212. The fairing 370 b may provide a smoothtransition between the tail rotor drive shaft cover 222 and theretreating side 232 of the tail boom 212. The fairing 370 b can besupplied as an aftermarket kit, along with strake(s) and/or vortexgenerators, and instructions 371 for installing such at the variouslocations described herein. Alternatively, the fairing may be part ofthe helicopter as manufactured and/or delivered by an OEM.

As can be seen in FIGS. 3A and 3B, the aerodynamic fairing 370 a, 370 beliminates the shoulder 215 b on the retreating side 232 of the tailboom 212. In some implementations, the upper or second strake 372 may beattached to the fairing 370 a, 370 b, before the fairing 370 a, 370 b isattached to the tail boom 212.

Altering a cross-sectional profile of a tail section 104 (FIGS. 1A, 1B)of a helicopter to make the retreating side 232 of the tail boom 212(the side opposite the approaching side 228) more aerodynamic in a waythat promotes flow attachment on the retreating side 232 of the tailboom 212 has many benefits. For example, altering the profile of theretreating side 232 of the tail boom 212 or tail section 104 can, amongother benefits: (i) remove downforce penalties resulting from adisadvantageously shape tail section profile; (ii) result in horsepowersavings by at least partially counteracting the reaction torque producedby rotation of the main rotor; and (iii) and can improve directionalcontrol by promoting a reduction in what is known as “loss of tail rotoreffectiveness” (in helicopters that include a main rotor that rotatescounterclockwise) or “uncommanded left yaw” (in helicopters that includea main rotor that rotates counterclockwise).

Such may include altering a profile of the tail section 104 to slow flowon the approaching side 228 of the tail boom 212 and speed up air flowon the retreating (i.e., opposite) side 232 of the tail boom 212 or tailsection 104. The alteration can be achieved by removing an existing tailrotor drive shaft cover 222 from the tail boom 215 and replacing it witha tail rotor drive shaft cover that has a more aerodynamic profile. Thealteration can also be achieved by adding structure to the tail boom 212or tail section 104. The resulting tail section 104 can have across-sectional profile that resembles an airfoil (e.g. having acontinuous cambered surface). The alteration may, or may not include theaddition of strakes 230 to the approaching side 228 of the tail boom 212and vortex generators 236 to the retreating side 232. Although some ofthe examples discussed herein relate to modifications of tail booms ortail sections that include a separate tail rotor drive shaft cover, theprinciples of the present disclosure are also applicable tomodifications of tail booms 212 or tail sections 104 in which the tailrotor drive shaft 222 is housed entirely within the tail boom 212. Insuch cases, the modification may include, for example, the addition ofat least one structural element to at least the opposite side of thetail boom 212 or tail section 104 to improve the aerodynamic profile ofthe tail boom 212.

FIG. 4A shows a pressure distribution around a conventional tail boom400 a and a tail rotor drive shaft cover 402 a of a helicopter produceusing computational fluid dynamics (CFD), the tail boom 400 a having anon-circular or oblong cross-sectional profile, and the tail sectionwithout strakes, vortex generators or fairing to produce an asymmetriccross-sectional profile.

As visible in FIG. 4A, the conventional tail boom 400 a develops asubstantial amount of low pressure on the approaching side 428 of thetail boom 400 a, and an even more substantial amount of low pressure onthe retreating side 432 of the tail boom. The lift generated by thedifference in pressure between the approaching and retreating sides 428,432 is a function of the distribution of pressures between theapproaching side 428 and the retreating side 432.

FIG. 4B shows a pressure distribution around a tail boom 212 and a tailrotor drive shaft cover 222 of a helicopter 300 b produced usingcomputational fluid dynamics (CFD), the tail boom 212 having anon-circular cross-sectional profile (FIG. 3B), and the tail section 104(FIGS. 1A, 1B) having a first strake 230 that extends along at least aportion of a length 126 (FIGS. 1A, 1B) of the tail boom 212 at a firststrake position 242 (FIG. 2C), a plurality of vortex generators 236 thatextend along at least a portion of the length 126 (FIGS. 1A, 1B) of thetail boom 212 at a vortex generator position 258 (FIG. 2C), and afairing 370 b attached to the tail boom 212 to provide the tail section104 (FIGS. 1A, 1B) with an asymmetric cross-sectional profile. Whilesimilar or even identical to that of FIG. 3B, a similar pressuredistribution would result from the configuration of FIG. 3A.

As visible in FIG. 4B, the inclusion of a properly positioned firststrake 230, vortex generators 236 and fairing 370 b significantlyincreases the lift generated by the difference in pressure between theapproaching and retreating sides 228, 232. As compared to FIG. 4A, thelow pressure on the approaching side 228 of the tail boom 212 issignificantly reduced by the structures, while the low pressure on theretreating side 232 is significantly enhanced. This change in pressuresubstantially aids in countering main rotor torque. This improvedpressure distribution results in horsepower savings by at leastpartially counteracting the reaction torque produced by rotation of themain rotor. It also improves directional control by reducing loss oftail rotor effectiveness.

FIG. 4C shows a pressure distribution around a conventional tail boom400 b and a tail rotor drive shaft cover 402 b of a helicopter produceusing computational fluid dynamics (CFD), the tail boom 400 b having acircular or oblong cross-sectional profile, and the tail section withoutstrakes, vortex generators or fairing to produce an asymmetriccross-sectional profile.

As visible in FIG. 4C, the conventional tail boom 400 b develops asubstantial amount of low pressure on the approaching side 428 of thetail boom 400 b, and an even more substantial amount of low pressure onthe retreating side 432 of the tail boom. The lift generated by thedifference in pressure between the approaching and retreating sides 428,432 is a function of the distribution of pressures between theapproaching side 428 and the retreating side 432.

FIG. 4D shows a pressure distribution around a tail boom 712 and a tailrotor drive shaft cover 222 of a helicopter 100 (FIGS. 1A, 1B) producedusing computational fluid dynamics (CFD), the tail boom 712 havingcircular cross-sectional profile (FIG. 7). The tail section 104 (FIGS.1A, 1B) has a first strake 230 that extends along at least a portion ofa length 126 (FIGS. 1A, 1B) of the tail boom 712 at a first strakeposition 742 (FIG. 7). The first strake position 742 is spacedapproximately 5 degrees to 15 degrees below a horizontal midplane 248 ofthe tail boom 712 on the approaching side 228, best illustrated in FIG.7. The tail section 104 (FIGS. 1A, 1B) optionally has a second strake772 at a second strake position 776. The second strake position 776 isproximate a position where the tail rotor drive shaft cover 222 joinsthe tail boom 712 (e.g., within a few inches). The tail section 104(FIGS. 1A, 1B) optionally has a third strake 792 at a third strakeposition 794. The third strake position 794 is proximate a vertical apex796 of the tail rotor drive shaft cover 222 or a fair 370 b. The tailsection 104 (FIGS. 1A, 1B) has a plurality of vortex generators 236 thatextend along at least a portion of the length 126 (FIGS. 1A, 1B) of thetail boom 712 at a vortex generator position 758 (FIG. 2C). The vortexgenerator position 758 is spaced approximately 5-15 degrees below ahorizontal midplane 248 of the tail boom 712 on the retreating side 232,best illustrated in FIG. 7, mirrored across a vertical axis 244 from thefirst strake 230. The tail section 104 (FIGS. 1A, 1B) optionally has afairing 370 b attached to the tail boom 712 to provide the tail section104 (FIGS. 1A, 1B) with an asymmetric cross-sectional profile. The tailboom 712 is similar to that illustrated in FIG. 7, although adds thethird strake 792.

As visible in FIG. 4D, the inclusion of a properly positioned firststrake 230, vortex generators 236 at first strake and vortex generatorpositions 742, 758, respectively, and addition of fairing 370 bsignificantly increases the lift on the retreating side 232 generated bythe difference in pressure between the approaching and retreating sides228, 232. Further addition of second strake 772 and/or third strake 792at second and third strake positions 776, 796, respectively may alsoincrease lift on the retreating side 232 relative to the conventionaltail boom 400 b. As compared to FIG. 4A, the low pressure on theapproaching side 228 of the tail boom 712 is significantly reduced bythe structures, while the low pressure on the retreating side 232 issignificantly enhanced. This change in pressure substantially aids incountering main rotor torque. This improved pressure distributionresults in horsepower savings by at least partially counteracting thereaction torque produced by rotation of the main rotor. It also improvesdirectional control by reducing loss of tail rotor effectiveness.

FIG. 5 shows a portion of a helicopter 500, according to one illustratedembodiment.

The helicopter 500 may be similar or even identical to the helicopter100 (FIGS. 1A and 1B), helicopter 200 (FIGS. 2A-2C), helicopters 300 a,300 b (FIGS. 3A, 3B). The helicopter 500 b includes many of the same orsimilar structures to that illustrated in FIGS. 2A-2C. Identical orsimilar are denominated with the same reference numbers as employed inFIGS. 2A-2C, 3A and 3B. Some structures are not specifically called outin FIG. 6 to enhance drawing legibility. Only significant differenceswith that of FIGS. 2A-2C, 3A and 3B are discussed below.

In contrast to the tail booms 212 illustrated in FIGS. 2A-2C, 3A and 3B,FIG. 5 shows a tail boom 512 that has a highly elliptical non-circularcross-sectional profile. As previously noted, for tail booms 512 havinghighly oblong or highly elliptical non-circular cross-sectionalprofiles, it is advantageous to employ a first strake 230 on theapproaching side 228 at a first strake position 242, a plurality ofvortex generators 236 on the retreating side 232 at a vortex generatorposition 258, a faring 370 a, 370 b, and to omit additional strakes inthe upper half 374 of the approaching side 228. The first strake 230,vortex generators 237, and fairing 370 b at least partially counteractmain rotor torque produced by the main rotor 106, and form as at leastpart of the anti-torque system.

The first strake 230, vortex generators 237, and fairing 370 b can besupplied as an aftermarket kit, along with instructions 371 (FIGS. 3A,3B) for installing such at the various locations described herein.Alternatively, the fairing may be part of the helicopter as manufacturedand/or delivered by an OEM.

FIG. 6 shows a portion of a helicopter 600, according to one illustratedembodiment.

The helicopter 600 may be similar or even identical to the helicopter100 (FIGS. 1A and 1B), helicopter 200 (FIGS. 2A-2C), helicopters 300 a,300 b (FIGS. 3A, 3B), and/or helicopter 500 (FIG. 5). The helicopter 600b includes many of the same or similar structures to that illustrated inFIGS. 2A-2C. Identical or similar are denominated with the samereference numbers as employed in FIGS. 2A-2C, 3A, 3B, and 5. Somestructures are not specifically called out in FIG. 6 to enhance drawinglegibility. Only significant differences with that of FIGS. 2A-2C, 3A,3B, and 5 are discussed below.

In contrast to the tail booms 212 illustrated in FIGS. 2A-2C, 3A and 3B,FIG. 6 shows a tail boom 612 that has a highly oblong non-circularcross-sectional profile. Portions of the tail boom 612 can have a radiusof curvature that is infinite or almost infinite, for instance theessentially vertical portions illustrated in FIG. 6 that extendimmediately above and below the horizontal axis or midplane 248. Aspreviously noted, for tail booms 612 having highly oblong or highlyelliptical non-circular cross-sectional profiles, it is advantageous toemploy a first strake 230 on the approaching side 228 at a first strakeposition 242, a plurality of vortex generators 236 on the retreatingside 232 at a vortex generator position 258, a faring 370 a, 370 b, andto omit additional strakes in the upper half 374 of the approaching side228. The first strake 230, vortex generators 237, and fairing 370 b atleast partially counteract main rotor torque produced by the main rotor106, and form as at least part of the anti-torque system.

The first strake 230, vortex generators 237, and fairing 370 b can besupplied as an aftermarket kit, along with instructions 371 (FIGS. 3A,3B) for installing such at the various locations described herein.Alternatively, the fairing may be part of the helicopter as manufacturedand/or delivered by an OEM.

FIG. 7 shows a portion of a helicopter 700, according to one illustratedembodiment.

The helicopter 700 may be similar or even identical to the helicopter100 (FIGS. 1A and 1B), helicopter 200 (FIGS. 2A-2C), helicopters 300 a,300 b (FIGS. 3A, 3B), helicopter 500 (FIG. 5) and/or helicopter 600(FIG. 6). The helicopter 700 b includes many of the same or similarstructures to that illustrated in FIGS. 2A-2C. Identical or similar aredenominated with the same reference numbers as employed in FIGS. 3A, 3B,5, and 6. Some structures are not specifically called out in FIG. 7 toenhance drawing legibility. Only significant differences with that ofFIGS. 2A-2C, 3A, 3B, 5, and 6 are discussed below.

In contrast to the tail booms 212, 512, and 612 illustrated in the FIGS.2A-2C, 5 and 6, the tail boom 712 has a circular cross-sectionalprofile. Since the tail boom 712 has a constant radius of curvature orconstant change in curvature of the surface of the tail boom 712,positioning of strake(s) and vortex generators will be specifieddifferently from those described above. Using CFD, applicant hasdetermined positions for the first strake, vortex generators, andoptionally second strake to enhance performance for a tail boom 712 witha circular cross-sectional profile.

A first strake 230 is positioned on an approaching side 228 of the tailboom 712, extending outwardly of the approaching side 228, along atleast a portion of a length 126 (FIGS. 1A, 1B) of the tail boom 712, ona lower half 240 of the tail boom 712 at a first strake position 742.

The first strake position 742 is located or positioned below ahorizontal midplane 248 of the tail boom 712, at an angle of fromapproximately 5 degrees to 15 degrees when viewed along a length of thetail boom 712, the angle measured between the horizontal midplane 248, acenter point or centerline 252 and a ray 755 that extends from thecenter point or centerline 252 to the first strake position 742. The ray755 represents 15 degrees from the horizontal midplane 248. A ray 754represents 5 degrees from the horizontal midplane 248.

A number of vortex generators 236 are positioned on an retreating side232 of the tail boom 712, extending outwardly of the approaching side,along at least a portion of a length 126 (FIGS. 1A, 1B) of the tail boom212, on the lower half 240 of the tail boom 712 at a vortex generatorposition 758.

The vortex generator position 758 is located or positioned below ahorizontal midplane 248 of the tail boom 712, at an angle of fromapproximately 5 degrees to 15 degrees when viewed along a length of thetail boom 712, the angle measured between the horizontal midplane 248, acenter point or centerline 252 and a ray 763 that extends from thecenter point or centerline 252 to the vortex generator position 758. Theray 763 represents 15 degrees from the horizontal midplane 248. A ray762 represents 5 degrees from the horizontal midplane 248.

The faring 370 b is optional, and could be either omitted, or replacedwith fairing 370 a (FIG. 3A). The faring 370 b provides the tail section104 (FIGS. 1A, 1B) with an asymmetrical cross-sectional profile, shapedto produce lift on the retreating side 228.

Optionally, a second strake 772 is positioned on the approaching side228 of the tail boom 712, extending along at least a portion of a length126 (FIGS. 1A, 1B) of the tail boom 212, on an upper half 374 of thetail boom 712 at a second strake position 776. The second strakeposition 776 is preferably within a few inches of where the fairing 370b joins the tail boom 712 on an upper portion 778 of the tail boom 212.The second strake 772 may be employed with tail booms having a circularcross-sectional profile or almost circular cross-sectional profiles(e.g., FIG. 3A), and which are not overly elongated or overly elliptical(e.g., FIG. 6). Applicants have found it advantageous to omit a secondstrake 772, at least for tail booms with overly elongated or overlyelliptical cross-sectional profiles. The second strake 772 adds weight,and tends to create a down force penalty, so any advantage in producingflow separation on the approaching side 228 is at least partially, ifnot wholly, offset.

The strake(s) 230, 772, vortex generators 236, and optional fairing 370b at least partially counteracts main rotor torque produced by the mainrotor 106, and constitutes at least part of the anti-torque system.

The strake(s) 230, 772, vortex generators 237, and fairing 370 b can besupplied as an aftermarket kit, along with instructions 371 (FIGS. 3A,3B) for installing such at the various locations described herein.Alternatively, the fairing may be part of the helicopter as manufacturedand/or delivered by an OEM.

FIG. 8 shows a method 800 manufacturing a helicopter or modifying a tailsection of a helicopter, according to one embodiment. The method 800 maybe employed during construction, fabrication or manufacturing of thehelicopter, for example by an original equipment manufacturer (OEM) orvendor of the OEM, prior to delivery of the helicopter to a purchaser orend user. Alternatively, the method 800 may be employed aftermanufacture or delivery of the helicopter by the OEM. For example, themethod 800 may be performed using an aftermarket retrofit kit, which mayinclude one or more strakes, vortex generators, fairings, andinstructions (e.g., textual, diagrams, graphics, audio, in written formor on nontransitory computer-readable media) for preforming the method.

At 802, a first strake is positioned to extend outwardly from theapproaching side of the tail boom at a first strake position. The firststrake position is dependent on a shape of the cross-sectional profileof the tail boom.

For example, for tail booms having non-circular cross-sectionalprofiles, the first strake position is located or positioned below thehorizontal midplane of the tail boom, at an angle of from approximately5 degrees to 15 degrees, when viewed along a length of the tail boom,above a location at which a change in the radius of curvature of theapproaching side below the horizontal plane is greatest, the anglemeasured between the location at which a change in the radius ofcurvature of the approaching side below the horizontal plane isgreatest, the center point or centerline and an axis that extends fromthe centerline to the first strake position.

Alternatively, for a tail boom with a non-circular (e.g., elliptical oroblong) cross-sectional profile, the first strake position ispositioned, oriented and/or located on the approaching side of the tailboom, on a lower half of the tail boom, such that an upstanding leg ofthe first strake is normal to the boom surface and from approximately 10degrees to approximately 16 degrees, or more preferably approximately 13degrees to approximately 14 degrees to the horizontal (e.g., horizontalaxis or midplane), where the upstanding leg is at an approximately rightangle to a base of the first strake, which attaches the first strake tothe tail boom.

For example, for tail booms having circular cross-sectional profiles,the first strake position is located or positioned below a horizontalmidplane of the tail boom, at an angle of from approximately 5 degreesto 15 degrees when viewed along a length of the tail boom, the anglemeasured between the horizontal midplane, a center point or centerlineand a ray that extends from the center point or centerline to the firststrake position.

At 804, the first strake attached or fixed on the approaching side ofthe tail boom at the first strake position. For example, the firststrake may be attached via fasteners, nut plates, rivets, or the like.

At 806, a plurality of vortex generators are positioned to extendoutwardly from the retreating side of the tail boom at a vortexgenerator position. The vortex generator position is dependent on ashape of the cross-sectional profile of the tail boom. For example, fortail booms having non-circular cross-sectional profiles, the vortexgenerator position is located or positioned below the horizontalmidplane of the tail boom, at an angle of from approximately 5 degreesto 15 degrees, when viewed along a length of the tail boom, above alocation at which a change in the radius of curvature of the approachingside below the horizontal plane is greatest, the angle measured betweenthe location at which a change in the radius of curvature of theapproaching side below the horizontal plane is greatest, the centerpoint or centerline and an axis that extends from the centerline to thevortex generator position. For example, for tail booms having circularcross-sectional profiles, the vortex generator position is located orpositioned below a horizontal midplane of the tail boom, at an angle offrom approximately 5 degrees to 15 degrees when viewed along a length ofthe tail boom, the angle measured between the horizontal midplane, acenter point or centerline and a ray that extends from the center pointor centerline to the vortex generator position.

At 808, the vortex generators are attached or fixed to the retreatingside of the tail boom at the vortex generator position. For example, thevortex generators may be attached via fasteners, nut plates, rivets, orthe like. The vortex generators can be attached or bonded to themodified tail boom individually; installed as a single assembly, such asa long strip with the vortex generator pattern integral to it, attachedto the boom: or as an integral part of the redesigned shaft cover orother aerodynamic fairing, that is attached to an unmodified tail boom.

For example, the vortex generators may be spaced a distance d(approximately 3 inches to 6 inches) apart at an angle α of about 10° toabout 30° off a vertical axis. By way of reference, a 0° placement wouldhave the long axis of the vortex generator perpendicular to thecenterline axis of the boom. The vortex generator placement off thevertical axis of 10° to 30° can result in the vortex generators beingcanted either forward or aft. The vortex generators can all be angledthe same relative to a vertical axis, or can alternate back and forthalong the tail boom as shown in FIG. 2B.

Optionally at 810, a second strake is positioned to extend outwardlyfrom the approaching side of the tail boom at a second strake position.The second strake position is located within two inches above or below alocation at which the drive shaft cover joins the approaching side ofthe tail boom. The inclusion of a second strake may be dependent on ashape of the cross-sectional profile of the tail boom. For example, fortail booms having circular cross-sectional profiles, a second or upperstrake may produce sufficient benefit to justify inclusion. For tailbooms having non-circular cross-sectional profiles, a second or upperstrake may produce sufficient benefit to justify inclusion if theparticular non-circular cross-sectional profile is not overly elongated.

Optionally at 812, the second strake is fixed to the approaching side ofthe tail boom at the second strake position. For example, the secondstrake may be attached via fasteners, nut plates, rivets, or the like.

Optionally at 814, a fairing is positioned on the retreating side of thetail boom at a fairing position to create an asymmetry between theapproaching and the retreating side of the tail boom. For example, thefairing may be positioned to extend from an apex of a tail rotor driveshaft cover, down to where the horizontal midplane of the tail boomintersects the retreating side of the tail boom.

Alternatively, optionally at 816, a tail rotor drive shaft cover isremoved. Optionally at 818, a fairing is positioned in place of theremoved tail rotor drive shaft cover to cover the tail rotor drive shaftat a fairing position to create an asymmetry between the approaching andthe retreating side of the tail boom.

Thus, modifying a tail boom of a helicopter may include removing a firstrotor drive shaft cover from the tail boom, in which the first rotordrive shaft cover including a first external surface that issubstantially symmetrical about a vertical plane of the tail boom wheninstalled on the tail boom; and installing a second rotor drive shaftcover on the tail boom, in which the second drive shaft cover includinga second external surface that is asymmetrical about the vertical planeof the tail boom when installed on the tail boom. The second externalsurface may include a plurality of vortex generators.

Optionally at 820, the fairing is fixed at the fairing position. Forexample, the fairing may be attached via fasteners, nut plates, rivets,or the like.

Although some of the examples relate to adding strake(s) and/or vortexgenerators to a tail boom that has been modified to include anasymmetrical profile, the present disclosure also encompasses applyingstrake(s) and/or vortex generators to a tail boom that includes anasymmetrical shape without additional modification. For example,applying vortex generators to the opposite side of helicopters withasymmetrical tail booms, such as the Augusta Westland AW169 and the Bell525 Relentless, can improve the performance of these aircraft. Further,the tail boom modifications disclosed herein can be incorporated into awide range of helicopters. As used herein, the term “helicopter”includes, without limitation, rotorcraft aircraft, rotary-wing aircraft,or other heavier-than-air aircraft that are lifted and sustained in theair horizontally by rotating wings or blades turning about a verticalaxes using power supplied by an engine. For example, helicoptersincluding the Bell UH-1, Bell Huey II, Sikorsky UH-60, and EurocopterHH-65A Dolphin helicopters are well suited for retrofitting with thetail boom systems disclosed herein. The various embodiments describedabove can be combined to provide further embodiments.

The embodiments, features, systems, devices, materials, methods andtechniques described herein may, in certain embodiments, be applied toor used in connection with any one or more of the embodiments, features,systems, devices, materials, methods and techniques disclosed in U.S.Provisional Patent Application Nos. 60/930,233, 61/816,507; 62/188,305and U.S. Pat. Nos. 4,708,305; 6,869,045; 7,063,289; 8,210,468;8,985,503; and 8,991,747. The above-mentioned U.S. Provisional PatentApplication Nos. 60/930,233; 61/816,507; 62/188,305 and U.S. Pat. Nos.4,708,305; 6,869,045; 7,063,289; 8,210,468; 8,985,503; and 8,991,747 arehereby incorporated by reference herein. Aspects of the embodiments canbe modified, if necessary to employ concepts of the various patents,applications and publications to provide yet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A helicopter, comprising: a fuselage; a main rotor coupled to thefuselage and that in operation rotates in a first rotational directionwith respect to the fuselage; a tail boom that extends rearwardly of thefuselage, the tail boom having an approaching side and a retreatingside, the approaching side being a side of the tail boom that the mainrotor approaches when rotating in the first rotational direction, theretreating side being a side of the tail boom that the main rotorretreats from when rotating in the first rotational direction, theretreating side opposite the approaching side across a width of the tailboom, the approaching side of the tail boom having a constant radius ofcurvature about a centerline, and the tail boom having an upper half, alower a half and a horizontal plane that extends between the upper halfand the lower half of the tail boom; and a strake that extends outwardlyfrom the approaching side of the tail boom, the strake positioned belowthe horizontal plane of the tail boom at an angle of from approximately5 degrees to 15 degrees, the angle measured between the horizontal planeand an axis that extends from the centerline to the strake.
 2. Thehelicopter of claim 1, further comprising: at least one vortex generatorthat extends outwardly from the retreating side of the tail boom, the atleast one vortex generator positioned below the horizontal plane of thetail boom at an angle of from approximately 5 degrees to 15 degrees, theangle measured between the horizontal plane and an axis that extendsfrom the centerline to the at least one vortex generator.
 3. Thehelicopter of claim 1 wherein the tail boom has a longitudinal axis, thestrake is a retrofit strake and extends parallel to the longitudinalaxis of the tail boom, and the vortex generator extends nonparallel tothe longitudinal axis of the tail boom.
 4. The helicopter of claim 1,further comprising: a tail rotor positioned proximate a distal end ofthe tail boom and that in operation rotates in a tail rotor rotationaldirection; a tail rotor drive shaft drivingly coupled to the tail rotor;and a tail rotor drive shaft cover that extends along the tail boom andwhich removably covers the tail rotor drive shaft.
 5. The helicopter ofclaim 4 wherein the strake is a first strake, the helicopter furthercomprising: a second strake that extends outwardly from the approachingside of the tail boom, the second strake positioned proximate a locationat which the tail rotor drive shaft cover joins the approaching side ofthe tail boom.
 6. The helicopter of claim 4 wherein the strake is afirst strake, the helicopter further comprising: a second strake thatextends outwardly from the approaching side of the tail boom, the secondstrake positioned within two inches above or below a location at whichthe tail rotor drive shaft cover joins the approaching side of the tailboom.
 7. The helicopter of claim 4, further comprising: a fairingcoupled on the retreating side of the tail boom to create an asymmetrybetween the approaching and the retreating side of the tail boom.
 8. Thehelicopter of claim 7 wherein the tail rotor drive shaft cover has anapex, and the fairing extends from the apex of the tail rotor driveshaft cover down to where the horizontal plane intersects the retreatingside of the tail boom.
 9. The helicopter of claim 8 wherein the fairingprovides a smooth transition between the apex of the tail rotor driveshaft cover and the retreating side of the tail boom.
 10. The helicopterof claim 1, further comprising: a fairing coupled on the retreating sideof the tail boom to create an asymmetry between the approaching and theretreating side of the tail boom.
 11. The helicopter of claim 10 whereinthe fairing forms a tail rotor drive shaft cover that removably coversthe tail rotor drive shaft.
 12. The helicopter of claim 1 wherein thetail boom tapers in thickness as the tail boom is traversed from thefuselage to a distal end of the tail boom, the tail boom extends in avertical plane of the fuselage at an non-right angle from the fuselage,and the horizontal plane of the tail boom is not horizontal to a surfaceof a ground or a horizontal plane of the fuselage.
 13. A method ofmanufacturing a helicopter having a fuselage, a main rotor coupled tothe fuselage and that in operation rotates in a first rotationaldirection with respect to the fuselage, and a tail boom that extendsrearwardly of the fuselage, the tail boom having an approaching side anda retreating side, the approaching side being a side of the tail boomthat the main rotor approaches when rotating in the first rotationaldirection, the retreating side being a side of the tail boom that themain rotor retreats from when rotating in the first rotationaldirection, the retreating side opposite the approaching side across awidth of the tail boom, the approaching side of the tail boom having aconstant radius of curvature about a centerline, and the tail boomhaving an upper half, a lower a half, and a horizontal plane thatextends between the upper half and the lower half of the tail boom, themethod comprising: positioning a strake to extend outwardly from theapproaching side of the tail boom at a strake position, the strakeposition located below the horizontal plane of the tail boom at an angleof from approximately 5 degrees to 15 degrees, the angle measuredbetween the horizontal plane and an axis that extends from thecenterline to the strake position; and fixing the strake to theapproaching side of the tail boom at the strake position.
 14. The methodof claim 13, further comprising: positioning at least one vortexgenerator to extend outwardly from the retreating side of the tail boomat a vortex generator position, the at least one vortex generatorposition located below the horizontal plane of the tail boom at an angleof from approximately 5 degrees to 15 degrees, the angle measuredbetween the horizontal plane and an axis that extends from thecenterline to the vortex generator position; and fixing the at least onevortex generator to the retreating side of the tail boom at the vortexgenerator position.
 15. The method of claim 13 wherein the strake is afirst strake, the strake position is a first strake position, thehelicopter further has a tail rotor positioned proximate a distal end ofthe tail boom and that in operation rotates in a tail rotor rotationaldirection, a tail rotor drive shaft drivingly coupled to the tail rotor,and a tail rotor drive shaft cover that extends along the tail boom andwhich removably covers the tail rotor drive shaft, the method furthercomprising: positioning a second strake to extend outwardly from theapproaching side of the tail boom at a second strake position, thesecond strake position located within two inches above or below alocation at which the tail rotor drive shaft cover joins the approachingside of the tail boom; and fixing the second strake to the approachingside of the tail boom at the second strake position.
 16. The method ofclaim 13, further comprising: positioning a fairing on the retreatingside of the tail boom at a fairing position to create an asymmetrybetween the approaching and the retreating side of the tail boom; andfixing the fairing at the fairing position.
 17. The method of claim 13wherein the helicopter further has a tail rotor positioned proximate adistal end of the tail boom and that in operation rotates in a tailrotor rotational direction, a tail rotor drive shaft drivingly coupledto the tail rotor; and a tail rotor drive shaft cover that extends alongthe tail boom and which removably covers the tail rotor drive shaft, thetail rotor drive shaft cover having an apex, and further comprising:positioning a fairing on the retreating side of the tail boom at afairing position in which the fairing extends from the apex of the tailrotor drive shaft cover down to where the horizontal plane intersectsthe retreating side of the tail boom.
 18. The method of claim 13 whereinthe helicopter further has a tail rotor positioned proximate a distalend of the tail boom and that in operation rotates in a tail rotorrotational direction, a tail rotor drive shaft drivingly coupled to thetail rotor, and a tail rotor drive shaft cover that extends along thetail boom and which removably covers the tail rotor drive shaft, thetail rotor drive shaft cover having an apex, the method furthercomprising: removing the tail rotor drive shaft cover; and positioning afairing at a fairing position in which the fairing covers the tail rotordrive shaft and extends down a portion of the retreating side of thetail boom.
 19. A method of retrofitting a helicopter having a tail boomwith an upper half, a lower a half, and a horizontal plane that extendsbetween the upper half and the lower half, the method comprising:attaching a strake to extend outwardly from an approaching side of thetail boom at a strake position, the strake position located below thehorizontal plane at an angle of from approximately 5 degrees to 15degrees.
 20. The method of retrofitting of claim 19, further comprising:positioning at least one vortex generator to extend outwardly from aretreating side of the tail boom, which is opposite the approachingside, at a vortex generator position, the vortex generator positionlocated below the horizontal plane at an angle of from approximately 5degrees to 15 degrees.
 21. The method of retrofitting of claim 20wherein the strake is a first strake and the strake position in a firststrake position, the method further comprising: positioning a secondstrake to extend outwardly from the approaching side at a second strakeposition, the second strake position located within two inches above orbelow a location at which a tail rotor drive shaft cover joins theapproaching side of the tail boom.
 22. The method of retrofitting ofclaim 20, further comprising: positioning a fairing on the retreatingside of the tail boom, which is opposite the approaching side, at afairing position to create an asymmetry between the approaching side andthe retreating side.
 23. The method of retrofitting of claim 22 whereinpositioning the fairing includes positioning the fairing to extend froman apex of the tail rotor drive shaft down to where the horizontal planeof the tail boom intersects the retreating side of the tail boom. 24.The method of retrofitting of claim 22, further comprising: removing atail rotor drive shaft cover; and positioning the fairing to cover atail rotor drive shaft and extend part way down the retreating side ofthe tail boom.